Field of the Invention:
The invention relates to a method for determining the NOx concentration in the exhaust gas from an internal combustion engine.
To measure the NOx concentration in a gas, for example, in the exhaust gas from an internal combustion engine, prior art devices use a thick-film measurement sensor. Such a measurement sensor is described, for example, in the publication N. Kato et al., xe2x80x9cThick Film ZrO2 NOx Sensor for the Measurement of Low NOx Concentrationxe2x80x9d, Society of Automotive Engineers, publication 980170, 1989, or in N. Kato et al., xe2x80x9cPerformance of Thick Film NOx Sensor on Diesel and Gasoline Enginesxe2x80x9d, Society of Automotive Engineers, publication 970858, 1997. The measurement sensor has two measurement cells and includes a zirconium oxide that conducts oxygen ions. It implements the following measurement concept: in a first measurement cell, to which the gas that is to be measured is supplied through a diffusion barrier, a first oxygen ion pump current is used to establish a first oxygen concentration, during which process there should be no decomposition of NOx. In a second measurement cell, which is connected to the first through a diffusion barrier, the oxygen content is reduced further by a second oxygen ion pump current. The decomposition of NOx at a measurement electrode leads to a third oxygen ion pump current, which is a measure of the NOx concentration. The entire measurement sensor is brought to an elevated temperature by an electrical heater, e.g., to 750xc2x0 C.
During the measurement of the NOx concentration, a deviation from the true NOx concentration arises because the NOx measured value is distorted as a result of slippage of oxygen from the first chamber into the second chamber. Although pumping the oxygen slippage in the second cell out reduces the oxygen content distorting the measurement signal considerably, it does not do so completely because oxygen that originates from the decomposition of NOx is still also recorded.
It is accordingly an object of the invention to provide a method for determining a NOx concentration that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that records a NOx concentration in the exhaust gas of an internal combustion engine using a measurement sensor in a more accurate way.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for determining the NOx concentration in an exhaust gas from an internal combustion engine including the steps of introducing some of the exhaust gas into a first measurement cell of a measurement sensor and establishing a first oxygen concentration in the first measurement cell through an oxygen ion pump current, establishing a second oxygen concentration in a second measurement cell connected to the first measurement cell, measuring the NOx concentration in the second measurement cell, determining from the oxygen ion pump current an air ratio signal representing a function of a X value of the exhaust gas, determining from the air ratio signal a measurement error representing a deviation of the measured NOx concentration from a true NOx concentration, and correcting the measured NOx concentration with the measurement error.
The invention is based on the discovery that the measurement error in the value for the NOx concentration supplied by a NOx measurement sensor that is of two-chamber structure is dependent on the level of the oxygen concentration in the exhaust gas from an internal combustion engine. The measurement sensor cannot measure the oxygen concentration. The first oxygen ion pump current, however, is a direct measure of the air ratio xcex of the exhaust gas. If the measurement error is now related not to the oxygen concentration, but rather to xcex, preferably to its reciprocal 1/xcex, it is found that the measurement error for many measurement sensors can be described by a simple function, and even substantially linearly for 1/xcex. The term substantially linearly is understood as meaning that, when developing a series of higher-order terms it is only necessary to take account of very low coefficients.
Therefore, an air ratio signal, preferably the 1/xcex value of the exhaust gas, is obtained from the first oxygen ion pump current in the first measurement cell, and, by such a signal, the measurement error is determined by using a relationship between air ratio signal and measurement error. The measured NOx concentration is then corrected by the measurement error.
The relationship is preferably determined in advance in a calibration measurement so that the measurement error is in the form of a characteristic diagram, a characteristic curve, or a functional relationship.
In accordance with another mode of the invention, a relationship between the air ratio signal and the measurement error is determined in a calibration measurement by exposing the measurement sensor to various gas mixtures, recording the measurement error and describing the measurement error as a characteristic diagram, a characteristic curve, or a functional relationship, and varying at least the xcex value and the NOx concentration in the gas mixtures.
In accordance with a further mode of the invention, the correction step is carried out by correcting with multiplication/addition calculation operations and/or subtraction calculation operations.
In accordance with an added mode of the invention, the measurement error determining step is carried out by applying a substantially linear function as a relationship between the air ratio signal and the measurement error.
To be able to obtain the air ratio signal from the first oxygen ion pump current, a characteristic curve, which represents the relationship between first oxygen ion pump current in the first cell and the air ratio signal, for example 1/xcex or another function of xcex, is also determined. In such a case, the relationship between first oxygen ion pump current and 1/xcex is, surprisingly, substantially linear, so that it is furthermore advantageous to select 1/xcex as the air ratio signal.
In accordance with an additional mode of the invention, a characteristic curve or a characteristic diagram is used to determine the air ratio signal from the oxygen ion pump current.
With the objects of the invention in view, there is also provided a method for determining the NOx concentration in an exhaust gas from an internal combustion engine including the steps of providing a measurement sensor having a first measurement cell and a second measurement cell connected to the first measurement cell, introducing some of the exhaust gas into the first measurement cell and establishing a first oxygen concentration in the first measurement cell through an oxygen ion pump current, establishing a second oxygen concentration in the second measurement cell, measuring the NOx concentration in the second measurement cell, determining from the oxygen ion pump current an air ratio signal representing a function of a xcex value of the exhaust gas, determining from the air ratio signal a measurement error representing a deviation of the measured NOx concentration from a true NOx concentration, and correcting the measured NOx concentration with the measurement error.
The method according to the invention has the advantage that only multiplications, subtractions, and additions are required during the correction of the measured NOx concentration. A division, which would impose excessively high demands on the calculation ability of an inexpensive microcontroller, is not required.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining the NOx concentration, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.